An Analysis of Near-Bottom Magnetic Anomalies: Sea-Floor Spreading and the Magnetized Layer

Abstract

Near-bottom magnetic data over six oceanic ridge segments in the East Pacific are inverted, giving magnetization solutions with alternate positive and negative bands which correspond to geomagnetic field reversals. We estimate the average half-width of the crustal formation zone to be 2-3 km, based on the transition widths between these bands. The solutions show a narrow region of high magnetization centred directly over the centre of spreading, superimposed on a more gradual decrease of magnetization amplitudes with age. Both features are attributed to weathering of highly magnetized pillow lavas. We demonstrate that the short wavelength ( < 3 km) anomalies are largely due to topography. Distances to reversal boundaries give distance us age curves for each ridge which show that spreading changes occur as sudden accelerations typically separated by several million years of very constant motion. These rate changes are probably accompanied by shifts in the locations of poles of relative motion, which are necessary in a system of more than two interacting plates. Palaeomagnetic data and reversal boundary locations from near-bottom and surface data are combined to give spreading half-rates and a refined time scale for the past 6 My. Widespread spreading rate variations occurred at 2-3 MyBP and about 5 MyBP, possibly as a response to large scale changes in the plate pattern.

Integrated magnetostratigraphy and biostratigraphy of the upper Pliocene-lower Pleistocene from the Monte Singa and Crotone areas in Calabria, Italy

“…The duration of 115 ka is considerably less than reported for the Olduvai subchron in the literature. Linear interpolation between the Gauss/Matuyama and the Matuyama/Brunhes boundary in deep-sea cores yields an average duration of 150 ka (1.91-1.76 Ma; [30,33]), whereas analysis of near-bottom marine magnetic anomalies yields a duration of 220 ka (1.88-1.66 Ma; [31,34,35]). The second value is closely approached by the duration of 200 ka (1.87-1.67 Ma) in the polarity time scale of Mankinen and Dalrymple [36], who apparently placed the boundaries at the oldest and youngest K/Ar ages of absolutely dated normal polarity observations.…”

Comment on “Tectonic rotations in extensional regimes and their paleomagnetic consequences for ocean basalts” by Kenneth L. Verosub and Eldridge M. Moores

“…The upper layer in young oceanic crust has been shown to be strongly magnetized on the basis of detailed magnetic surveys [Talwani et al, 1971;Atwater and Mudie, 1973] and from dredge and DSDP sampling [Irving, 1970;Lowrie, 1977]. Decay of the upper layer contribution has been interpreted from the decrease in anomaly amplitude near the ridge crest [Klitgord et al, 1975] and from the apparent increase with time in transition zone widths [Blakely, 1976] Table 1 for parameters. tic [Cande, 1978]. It is difficult to model such a large phase shift with the two-layer model with no tilting; geomagnetic field behavior was thought to be responsible.…”

The influence of magma supply and eruptive processes on axial morphology, crustal construction and magma chambers

“…Such a narrow dike intrusion width is supported by independent statistical observations of the proportion of dikes in ophiolites that exhibit one-sided cooling, due to having been split by a subsequent dike [1,17,18]. Studies of marine magnetic anomaly reversal widths and variability [4][5][6][7][8] suggest a relatively wide zone of lava emplacement, 1-3 km wide. However, these data can not resolve the relative contributions of the width of the intrusion zone and lava flow distribution to the lava emplacement process.…”

“…Near-bottom magnetic profiles from ridges in the Pacific covering intertnediate to fast spreading rates give an average half-width of crustal fortnation of 2-3 km [8]. Measurements made by Macdonald et al, [7] on the EPR 21°N give transition widths of 1-1.4 km.…”

“…These two magmatic processes generate an upper crustal structure composed of lavas piled on top of one another, underlain by a sheeted dike complex. Constraints on the processes of upper crustal construction have come from ophiolite studies [1 ,2], observations in active rifts (e.g., Iceland [3D, modeling of marine magnetic anomaly shapes and transition thicknesses [4][5][6][7][8], mapping [9][10][11][12][13][14], dating [15], and drilling (e.g. DSDP Hole 504B [I6D of young ocean crust.…”